Process for producing an ion exchange membrane by grafting non ion-selective monomers onto a ion exchange

ABSTRACT

A separator membrane for use in secondary alkali batteries having a non-ion selective hydrophilic group and ion exchange group said membrane having an electrical resistance which, when said membrane is heated in 12NaOH for 30 minutes at a temperature between about 20° and 100° C., varies from the initial value for 20° C. by an amount between 10% and about 24% and the process thereof are herein disclosed.

CROSS REFERENCE TO OTHER APPLICATION

This application is a continuation of co-pending application Ser. No.867,582, filed May 27, 1986, now abandoned, itself a continuation ofapplication Ser. No. 647,667, filed Sept. 6, 1984, now abandoned, itselfa division of application Ser. No. 551,884, filed Nov. 15, 1983, nowabandoned, itself a continuation-in-part application of Ser. No.290,302, filed Aug. 5, 1981, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a separator membrane for use insecondary alkali batteries. More particularly, the present inventionrelates to a separator membrane for use in secondary alkali batteriesthat is dimensionally stable and which retains a constant electricalresistance in a hot and concentrated aqueous alkali solution.

2. Description of the Prior Art

Secondary alkali batteries (e.g. Ni--Zn and Ni--Cd) use a concentratedKOH or NaOH aqueous solution as the electrolyte and require a separatorthat has to meet essentially the same requirements as a separator usedin silver oxide primary batteries. In secondary alkali batteries, thetemperature of the electrolyte may exceed 100° C. if the battery isrepeatedly charged and discharged at high rate within a short period.If, under this hot environment, the electrical resistance of theseparator is increased or its dimensions are changed, the batteriesperformance is reduced.

Therefore, the present inventors have made various efforts to produce aseparator membrane which experiences minimum swelling in a hot andconcentrated alkaline aqueous solution.

Ion exchange membranes are important as a separator indispensable toelectrolysis and electrodialysis. Most of the conventional ion exchangemembranes are composed of a styrene-divinyl benzene or astyrene-butadiene copolymer having a sulfone group or a quaternary vinylpyridinum salt introduced therein, but because of their inadequatemechanical strength and chemical resistance, they find only limitedutility.

With recent advances in technology for manufacture and application ofion exchange membranes, various ion exchange membranes have beendeveloped and their use is increasingly expanded. An ion exchangemembrane made of a fluorine-containing polymer using a sulfone group orcarboxyl group as an exchange has been found to be useful as a separatorfor brine electrolysis in the manufacture of caustic soda or as ahigh-molecular electrolyte for water electrolysis, and it has also beenfound that an ion exchange membrane prepared by grafting acrylic acid ormethacrylic acid onto a polyethylene film has good properties for use asa separator in an alkali batteries using aqueous KOH or NNaOH as anelectrolyte. But one great problem of these ion exchange membrane isthat their electrical resistance is increased with increasingtemperature in a relatively high concentration of aqueous KOH or NaOH.To reduce the theoretical decomposing voltage and also to lower theoverpotential and ohmic loss due to solution resistance, brineelectrolysis or water electrolysis is desirably performed at elevatedtemperatures. The temperature of secondary batteries is rapidlyincreased during high rate discharge and charge cycles, so they arerequired to have high heat resistance. So the separator for use in theseapplications is desirably such that its electrical resistance is notincreased at elevated temperatures.

SUMMARY OF THE INVENTION

We have reviewed many cases where the electrical resistance of separatormembrane is increased with temperature in a highly concentrated aqueousalkali solution, and have found that the higher the exchange capacity ofthe membrane, the greater the increase in electrical resistance, andthat the increase in electrical resistance is due to a drop in thepercentage swelling of the membrane that occurs with increasingconcentrations and temperatures of the aqueous alkali. Based on thisfinding, we have made various studies to develop an ion exchangemembrane whose percent swelling is not greatly influenced by thetemperature or the concentration of the solute and accomplished aninvention which is described herein.

Therefore, the primary object of this invention is to provide aseparator membrane whose electrical resistance depends little on thetemperature of a highly concentrated aqueous alkaline solution. Thisobject is achieved by introducing both a non-ion selective hydrophilicgroup and an ion exchange group in the substrate of the monomer.

Another object of this invention is to provide a separator membrane foruse in secondary alkali batteries having a non-ion selective hydrophilicgroup and ion exchange group, said membrane having an electricalresistance which, when said membrane is heated in 12N NaOH for 30minutes at a temperature between about 20° and 100° C., varies from theinitial value for 20° C. by an amount between 10% and about 24%.

A further object of this invention is to provide a process for producinga separator membrane for use in secondary alkali batteries having anon-ion selective hydrophilic group and an ion exchange group, whichprocess comprises grafting to an ion exchange membrane a monomer havinga non-ion selective hydrophilic group and/or a monomer having afunctional group capable of being converted to a non-ion selectivehydrophilic group by hydrolysis or other suitable treatment, and thenoptionally performing hydrolysis or other suitable treatment to therebyintroduce a non-ion selective hydrophilic group into the ion exchangemembrane, whereby the resultant membrane has an electrical resistancewhich, when said membrane is heated in 12N NaOH for 30 minutes at atemperature between about 20° and 100° C., varies from the initial valuefor 20° C. by an amount between 10% and about 24%.

A still further object of this invention is to provide a process forproducing a separator membrane for use in secondary alkali batterieshaving a non-ion selective hydrophilic group and an ion exchange group,wherein a monomer having an ion exchange group and/or a monomer having afunctional group capable of being converted to an ion exchange group byhydrolysis or other suitable treatment, as well as a monomer having anon-ion selective hydrophilic group and/or a monomer having a functionalgroup capable of being converted to a non-ion selective hydrophilicgroup by hydrolysis or other suitable treatment are co-grafted orindividually grafted to a polymer film, and optionally the polymer filmis hydrolyzed or subjected to other suitable treatment at a suitabletiming to thereby introduce an ion exchange group and a non-ionselective hydrophilic group into the polymer film, whereby the resultantmembrane has an electrical resistance which, when said membrane isheated in 12N NaOH for 30 minutes at a temperature between about 20° and100° C., varies from the initial value for 20° C. by an amount between10% and about 24%.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The substrate in which a non-ion selective hydrophilic group or an ionexchange group is introduced is made of various hydrocarbon- orhalogen-containing polymer films (the term "film" as used hereinincludes "sheets") or ion exchange membrane prepared by thecopolymerization method or blending method. The ion exchange membranecontemplated by this invention can be produced by grafting a monomercontaining as ion exchange group to the hydrocarbon orhalogen-containing polymer film, or by grafting a monomer containing anon-ion selective hydrophilic group to the ion exchange membrane of thelatter type.

In the practice of this invention, the following methods can be used toproduce an ion exchange membrane wherein both an ion exchange group anda non-ion selective hydrophilic group are incorporated in the substrate:

(1) A monomer containing an ion exchange group (hereunder referred to as"monomer A") and/or a monomer containing a functional group capable ofbeing converted to an ion exchange group by treatment such as hydrolysis(hereunder referred to as "monomer B") and a monomer containing anon-ion selective hydrophilic group (hereunder referred to as "monomerC") and/or a monomer having a functional group capable of beingconverted to a non-ion selective hydrophilic group by treatment such ashydrolysis (hereunder referred to as "monomer D") are co-grafted to apolymer film;

(2) monomer A and/or monomer B are first grafted to a polymer film, andthen monomer C and/or monomer D are grafted to the film;

(3) monomer C and/or monomer D are first grafted to a polymer film, andthen monomer A and/or monomer B are grafted to the film; and

(4) monomer C and/or monomer D are grafted to an ion exchange membrane.

Hydrolysis and other treatments to convert the functional group inmonomer B to an ion exchange group or to convert the functional group inmonomer D to a non-ion selective hydrophilic group may be performed inthe last step of each method, or after the first grafting is effected inmethods (2) and (3). The timing of this treatment is properly selecteddepending upon the physical properties of the monomer and solvent used,as well as the ease of the intended grafting.

The substrate for the ion exchange membrane of this invention is made ofa film of hydrocarbon- or halogen-containing polymers to which a monomercan be grafted, such as polyethylene polypropylene, polybutadiene,polytetrafluoroethylene, poly(tetrafluoroethylene-ethylene),poly(tetrafluoroethylenehexafluoropropylene),poly(tetrafluoroethylene-perfluorovinylether), polyvinylidene fluoride,polyvinyl fluoride, polyethylene terephthalate, polyvinyl chloride,polyamide and polyester. Alternatively, the substrate may be made of anion exchange membrane to which a monomer can be grafted and which isprepared by treating the above listed polymer films by the graftpolymerization, copolymerization or blending method.

Examples of monomer A are vinyl compounds containing a sulfone group orcarboxyl group such as styrenesulfonic acid, vinylsulfonic acid,trifluorovinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid,acrylic acid, trifluoroacrylic acid, methacrylic acid, or sodium andpotassium salts thereof. Examples of monomer B are vinyl compoundshaving a functional group capable of being converted to an ion exchangegroup such as sulfone group or carboxyl group by such treatment ashydrolysis, for instance, trifluorovinylsulfonyl fluoride, methyltrifluoro-acrylate, methyl or ethyl esters of acrylic acid ormethacrylic acid, acrylamide and acrylonitrile. Examples of monomer Care hydrophilic vinyl compounds containing a hydroxyl group, glycol,etc. such as 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, alkylalcohol, polyethylene glycol acrylate, polyethylene glycol methacrylate,polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, andN-vinylpyrrolidone. Examples of monomer D are vinyl monomers having afunctional group that is converted to a non-ion selective hydrophilicgroup by such treatment as hydrolysis after grafting, for instance,vinyl acetate, allyl acetate and allyl formate. The proportion ofmonomer A and/or monomer B to monomer C and/or monomer D to beintroduced into the polymer, namely, the ratio of the concentration ofion-exchange groups (exchange capacity) to that of non-ion selectivehydrophilic group in the separator membrane, is determined properly bythe object of the use of the membrane. The electrical resistance of theseparator membrane primarily depends on the exchange capacity andpercent swelling, and it decreases as these factors increase. So, evenif the exchange capacity is small, an ion exchange membrane with ionelectrical resistance can be produced by increasing the concentration ofhydrophilic groups. But if the ion exchange membrane for use as aseparator for electrolysis or electrodialysis contains an excessivelygreat amount of non-ion selective hydrophilic group, the ion selectivityis decreased and hence the current efficiency is decreased. If the ionexchange membrane is used as a separator in batteries which does notrequire very high ion permselectivity, the content of the non-ionselective hydrophilic group is preferably increased to prevent theelectrical resistance of the separator from being increased withincreasing temperatures in a highly concentrated aqueous alkalinesolution In alkaline secondary cells, OH⁻ is spent at the negativeelectrode during discharge and at the positive electrode duringcharging, so in order to achieve high rate discharge and charge, OH⁻must be supplied through the separator. So, the separator is preferablymade of a membrane having a high concentration of non-ion selectivehydrophilic groups rather than an ion selective exchange group such assulfone group or carboxyl group.

Grafting of the monomers listed above can be performed by either theradiation grafting method using ionizing radiation or the catalyticmethod using a radical initiator. The object of this invention can beachieved more readily by the former method. The radiation graftingmethod can be accomplished by either pre-irradiation or simultaneousirradiation. According to the pre-irradiation, a polymer film or ionexchange membrane is irradiated with ionizing radiation, and then it isimmersed in or contacted by a monomer solution for grafting of themonomer. According to the simultaneous irradiation, a polymer film orion exchange membrane within or in contact with a monomer solution isirradiated with ionizing radiation for grafting the monomer. Eithermethod can be used to achieve the object of this invention.

When monomer B and/or monomer D is used in this invention, thefunctional group in monomer B must be converted to an ion exchange groupand that in monomer D to non-ion selective hydrophilic group by asuitable treatment such as hydrolysis. The proper method of treatmentshould be determined depending on the properties of the polymer film orion exchange membrane used as the substrate, and on the graft polymer(i.e. homopolymer or copolymer of monomer B or D).

This invention is now described in greater detail by reference to thefollowing examples to which the scope of this invention is by no meanslimited.

EXAMPLE 1

A commercial ion exchange membrane, Nafion 120 (available from du Pont)was irradiated with 5 Mrad of electron beams from a resonant transformer(2 MV. 1 mA) in a nitrogen atmosphere. The Nafion 120 was transferred toa reactor which was evacuated to 10⁻⁴ mmHg and fed with a solution ofN-vinylpyrrolidone in benzene (40 wt % conc.) that had been bubbled withnitrogen gas to reduce the concentration of dissolved oxygen to lessthan 1 ppm. The Nafion 120 thus immersed in N-vinylpyrrolidone (WAKOPURE CHEMICAL INDUSTRIES CO., LTD.) was left to stand for 5 hours atroom temperature to graft the monomer to the Nafion 120. To remove theunreacted monomer and homopolymer (non-graft polymer) by extraction, theresulting membrane was taken out of the reactor and washed, in sequence,with benzene (60° C.) for 5 hours, acetone at room temperature for 30minutes and boiling distilled water for 5 hours, and dried. Theresulting graft membrane had a graft ratio of 43 % as determined by theweight method.

The graft membrane (NVP-g-Nafion 120) and Nafion 120 as commerciallyavailable were heated in 12 N NaOH at a temperature between 20° and 100°C. for 30 minutes, and the electrical resistance of each membrane wasmeasured using: an a.c. source (1,000 Hz). The results are shown inTable 1.

                  TABLE 1                                                         ______________________________________                                        Electrical Resistance (in ohm-cm.sup.2)                                                 Heating temperature (°C.)                                    Membrane    20       40     60     80   100                                   ______________________________________                                        NVP-g-Nafion 120                                                                          19.4     20.2   25.4   22.0 23.2                                  Nafion 120  25.1     30.2   44.4   63.4 96.3                                  ______________________________________                                    

The percentage change of the electrical resistance of the ion exchangemembrane with the rise in temperature from 20° C. to 100° C., calculatedfrom Table 1 above, is 19.5/8%.

EXAMPLE 2

Commercially available Nafion 120 was immersed in a solution of vinylacetate (WAKO PURE CHEMICAL INDUSTRIES CO., LTD.) in acetone (35% conc)and by repeating freeze-degassing three (10⁻⁴ mmHg), the concentrationof dissolved oxygen was reduced less than 0.5 ppm. The nafion wasirradiated with γ-rays from Co-60 at a dose rate of 1×10⁵ rad/hr at roomtemperature for 3 hour to graft vinyl acetate to the polymer. Theresulting graft membrane was washed thoroughly with boiling acetone anddried. The membrane had a graft ratio of 25%. THE membrane was thenhydrolyzed with a mixture of 60 parts of methanol, 38 parts of water and2 parts of NaOH at 80° C. for 30 minutes. The hydrolyzed membrane washeated in 12N NaOH for 30 minutes as Example 1, and its electricalresistance was measured at room temperature. The results are set forthin Table 2.

                  TABLE 2                                                         ______________________________________                                        (in ohm-cm.sup.2)                                                             ______________________________________                                        Heating temperature (°C.)                                                             20      40     60    80   100                                  Electrical resistance                                                                        18.6    18.5   19.3  22.5 21.3                                 ______________________________________                                    

The percentage change of the electrical resistance of the ion exchangemembrane with the rise in temperature from 20° C. to 100° C., calculatedfrom Table 2 above, is 14.5%.

EXAMPLE 3

A high-density polyethylene (ASAHI CHEMICAL "F 400") membrane 50 μmthick was irradiated with 30 Mrad of electron beams as in Example 1,transferred to a monomer mixture (35% sodium trifluoroacrylate and 65%alkyl alcohol) prepared in accordance with U.S. Pat. No. 2,795,601, J.L. Rendall et al. that had been freeze-degassed to reduce the content ofdissolved oxygen to less than 0.5 ppm, and immersed there at 35° C. for5 hours for grafting the monomers to the polyethylene. The resultingmembrane was then washed by immersing it in a mixture of 70 parts ofmethanol and 30 parts of water at 80° C. for 10 hours, and dried. Themembrane had a graft ratio of 45% as determined by the weight method.

Three samples were taken out of the so obtained membrane and they wereimmersed in 12 N NaOH at room temperature (25° C.), 60° C. and 100° C.for 30 minutes, and their electrical resistance (specific resistance)was measured at room temperature in 12 N NaOH. The respective valueswere 35, 38 and 37 ohm-cm.

EXAMPLE 4

A polytetrafluoroethylene (NITO DENKO "NITOFLON No. 900") membrane 80 μmthick was immersed in a 50% aqueous solution of acrylic acid (KISHIDACHEMICAL) and bubbled with nitrogen to reduce the content of dissolvedoxygen to less than 0.5 ppm, and irradiated with γ-rays from Co-60 at adose rate of 1×10⁵ rad/hr at room temperature for 5 hours in a nitrogenatmosphere. The irradiated membrane was washed with water and dried. Theresulting graft membrane had a graft ratio of 34%. Part of the membranewas cut out and immersed in a 20% aqueous solution of polyethyleneglycol dimethacrylate (SHIN NAKAMURA CHEMICAL CO., LTD. "A-9G") ##STR1##with γ-rays from Co-60 as in above. The resulting membrane had a graftratio of 12%.

The electrical resistance (specific resistance) of the two membranes(Membrane I: only acrylic acid was grafted, Membrane II: acrylic acidand polyethylene glycol dimethacrylate were grafted) was measured as inExample 3.

                  TABLE 3                                                         ______________________________________                                        Electrical Resistance                                                         (Specific Resistance) (in ohm-cm)                                             ______________________________________                                        Heating temperature (°C.)                                                               25         60    100                                         Membrane I       45         70    110                                         Membrane II      39         42     45                                         ______________________________________                                    

The percentage change of the electrical resistance of the ion exchangemembrane with the rise in temperature from 25° C. to 100° C., calculatedfrom Table 3 above, is 15.3/8%.

EXAMPLE 5

A low-density polyethylene (ASAHI DOW "F-2135") film 50 μm thick wasirradiated with 10 Mrad of electron beams and immersed in a 50% aqueoussolution of acrylic acid (KISHIDA CHEMICAL) (O₂ conc 0.3 ppm) for 3hours to provide a membrane having a graft ratio of 65% (Membrane I).Part of the membrane was irradiated with γ-rays from Co-60 at it wasimmersed in 20% vinyl acetate in acetone. The graft membrane was washedwith acetone at 50° C. for 10 hours, and dried. The resulting membranehad a graft ratio of 22%.

The electrical resistance (specific resistance) of the two membranes wasmeasured as in Example 3 except that 40% KOH was used as an electrolyte.The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Electrical Resistance                                                         (Specific Resistance) (in ohm-cm)                                             ______________________________________                                        Heating temperature (°C.)                                                               25         60    110                                         Membrane I       18         27    53                                          Membrane II      14         13    16                                          ______________________________________                                    

The transport member of Membranes I and II was measured with 0.5N KCland 0.1N KCl, and the respective values were 0.79 and 0.63.

EXAMPLE 6

A polytetrafluoroethylene (NITO DENKO "NITOFLON No. 900") film 50 μmthick was irradiated with 3 Mrad of electron beams in a nitrogenatmosphere, and immersed in a solution of freeze-degassed vinyl acetate(MITSUBISHI RAYON) in acetone (25%) at room temperature for 2 hours tograft the vinyl acetate to the polymer. The resulting membrane had agraft ratio of 18%. The membrane was then hydrolyzed in Example 2. Thehydrolyzed membrane was irradiated with 3 Mrad of electron beams andimmersed in an aqueous solution of methacrylic acid (conc. 25%) at roomtemperature for 5 hours. The solution had been degassed to reduce theconcentration of dissolved oxygen to less than 0.5 ppm. The resultingmembrane had a graft ratio of 53%. The electrical resistance of thegraft membrane was measured as in Example 3. The results are shown inTable 5.

                  TABLE 5                                                         ______________________________________                                        (in ohm-cm)                                                                   ______________________________________                                        Heating temperature (°C.)                                                               25         60    100                                         Electrical resistance                                                                          20         22     22                                         (specific resistance)                                                         ______________________________________                                    

The percentage change of the electrical resistance of the ion exchangemembrane with the rise in temperature from 25° C. to 100° C., calculatedfrom Table 5 above, is 10.0%.

EXAMPLE 7

A polytetrafluoroethylene (NITO DENKO "NITOFLON No. 900") film 80 μmthick was immersed in a mixture of 40 parts of methyl trifluoroacrylateand 60 parts of allyl alcohol which was bubbled with nitrogen to reducethe concentration of dissolved oxygen to less than 0.5 ppm, and then thefilm was irradiated with γ-rays from Co-60 at a dose rate of 1×10 rad/hrfor 5 hours. To remove any unreacted monomer and homopolymer, the filmwas refluxed in boiling dimethyl sulfoxide for 10 hours to provide amembrane having a graft ratio of 48%. The graft membrane was hydrolyzedas in Example 2 and the electrical resistance (specific resistance) ofthe membrane was measured as in Example 3. The results are shown inTable 6.

                  TABLE 6                                                         ______________________________________                                        (in ohm-cm)                                                                   ______________________________________                                        Heating temperature (°C.)                                                               25         60    100                                         Electrical resistance                                                                          35         33     33                                         (specific resistance)                                                         ______________________________________                                    

EXAMPLE 8

A high-density polyethylene (ASAHI CHEMICAL "F400") film 25 μm thick wasirradiated with 20 Mrad of electron beam and immersed in a 50% aqueoussolution of freeze-degassed acrylamide at room temperature for 5 hoursto provide a membrane having a graft ratio of 63%. The membrane was thenhydrolyzed with a 2.5% aqueous solution of KOH at 95° C. for 30 minutes.The resulting membrane was designated as Membrane I. Part of Membrane Iwas cut out and irradiated with γ-rays from Co-60 at a dose rate of1×10⁵ rad/hr at room temperature for 3 hours as it was immersed in asolution of N-vinylpyrrolidone in benzene (conc. 30%). After refluxingin boiling benzene, the irradiated membrane was dried to provide a graftmembrane having a graft ratio of 18% (Membrane II). The electricalresistance (specific resistance) and transport number of the twomembranes were measured as in Example 5. The results are shown in Table7.

                  TABLE 7                                                         ______________________________________                                        Electrical Resistance                                                         (specific resistance) (ohm-cm)                                                            Temperature (°C.)                                                                  Transport                                             Membrane      25    60       110  number                                      ______________________________________                                        I (acrylamide 15    28       45   0.89                                        grafted)                                                                      II (acrylamide +                                                                            13    13       16   0.75                                        N-vinylpyrro-                                                                 lidone grafted)                                                               ______________________________________                                    

The percentage change of the electrical resistance of the ion exchangemembrane with the rise in temperature from 25° C. to 110° C., calculatedfrom Table 7 above, is 23.07%.

EXAMPLE 9

A poly(tetrafluoroethylene-hexafluoropropylene) (du Pont "FED Teflon")film 80 μm thick was irradiated with 5 Mrad of electron beams at roomtemperature in a nitrogen atmosphere, and immersed in a mixture of 40parts of methyl trifluoroacrylate, 10 parts of allyl acetate and 50parts of tetrahydrofuran at room temperature for 24 hours. The mixturehad been bubbled with nitrogen to reduce the concentration of dissolvedoxygen to less than 0.5 ppm. After the grafting, the film was taken outof the mixture and washed with a boiling mixed solvent of 50 parts oftetrahydrofuran and 50 parts of acetone for a whole day and night, anddried. The resulting membrane had a graft ratio of 47%. The membrane wasthen heated in a mixture of 50 parts of dimethyl sulfoxide, 47.5 partsof water and 2.5 parts of NaOH at 80° C. for one hour, and theelectrical resistance (specific resistance) of the membrane was measuredas in Example 3. The results are shown in Table 8.

                  TABLE 8                                                         ______________________________________                                        (in ohm-cm)                                                                   ______________________________________                                        Heating temperature (°C.)                                                               25         60    100                                         Electrical resistance                                                                          45         42     40                                         (specific resistance)                                                         ______________________________________                                    

EXAMPLE 10

A high-density polyethylene (ASAHI CHEMICAL "F400") film 75 μm thick wasimmersed in a mixture of 30 parts of vinyl acetate (MITSUBISHI RAYON),50 parts of methanol and 20 parts of water, and bubbled with nitrogen toreduce the concentration of dissolved oxygen to less than 0.5 ppm.Thereafter, the film was irradiated with γ-rays from Co-60 at a doserate of 1×10⁵ rad/hr for 5 hours. The resulting membrane was washed withboiling acetone and dried. The membrane had a graft ratio of 43.3%. Themembrane was subsequently irradiated with 10 Mrad of electron beams andimmersed in a nitrogen-bubbled mixture of 50% acrylonitrile and 50%acetone at 40° C. for 2 hours to graft the acrylonitrile to themembrane. The membrane was washed and dried as above to provide a graftratio of 67.5%. The membrane to which vinyl acetate and acrylonitrilewere thus grafted was heated in a mixture of 60 parts of methanol, 37.5parts of water and 2.5 parts of NaOH at 80° C. for 60 minutes. Theelectrical resistance (specific resistance) of the membrane was measuredas in Example 3, and the results are shown in Table 9.

                  TABLE 9                                                         ______________________________________                                        (in ohm-cm)                                                                   ______________________________________                                        Heating temperature (°C.)                                                               25         60    100                                         Electrical resistance                                                                          23         22     20                                         (specific resistance)                                                         ______________________________________                                    

EXAMPLE 11

A polytetrafluoroethylene (NITO DENKO "NITFLON No. 900") film 50 μmthick was immersed in a mixture of 30 parts of trifluorovinylsulfonylfluoride, 30 parts of allyl acetate and 40 parts of Freon R-113 preparedby Journal Chemical Society, C-1966, p.1171, R. E. Banks, and afterbubbling with nitrogen, the film was irradiated with γ-rays from Co-60at a dose rate of 1×10⁵ rad/hr for 10 hours. Then, the irradiated filmwas washed with a boiling mixture of 50 parts of Freon R-113 and 50parts of acetone, and dried. The resulting membrane had a graft ratio of53.2%. The membrane was hydrolyzed as in Example 2, and the electricalresistance (specific resistance) of the membrane was measured as inExample 1. The results are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                        (in ohm-cm)                                                                   ______________________________________                                        Heating temperature (°C.)                                                                25         60    100                                        Electrical resistance                                                                          125        127    119                                        (specific resistance)                                                         ______________________________________                                    

What is claimed is:
 1. A process for producing a separator membrane foruse in secondary alkali batteries having a non-ion selective hydrophilicgroup and an ion exchange group comprising grafting to the ion exchangemembrane a monomer selected from the group consisting ofN-vinylpyrrolidone and vinyl acetate onto a Nafion substrate, wherebythe resultant membrane has an electrical resistance which, when saidmembrane is heated in 12N NaOH for 30 minutes at a temperature between20° C. and 100° C., varies from the initial value for 20° C. by anamount between 10% and 24% and wherein the swelling percent of themembrane is below 20% at a temperature of less than 10° C.
 2. A processfor producing a secondary alkali battery having a separator membrane,comprising the step of contacting the resultant membrane of claim 7 withan alkali solution of a secondary alkali battery so as to provide asecondary alkali battery having said resultant membrane as a separatormembrane.